Conventionally, sheet processing apparatuses that inspect various kinds of sheet have been practically used. The sheet processing apparatuses each include an image reading apparatus that detects fluorescence and afterglow from sheet.
The sheet has a pattern that is printed with fluorescent ink containing a fluorophore. The fluorophore is excited by light (excitation light) having a predetermined wavelength, such as UV light, and enters a state of emitting light (fluorescence). When excitation light irradiation ends, the fluorophore enters a state of emitting light (afterglow) that gradually attenuates with time. The image reading apparatuses read a fluorescence image, which is an image of fluorescence, at a timing of irradiating the fluorophore with the excitation light, and read an afterglow image, which is an image of afterglow, in a state where irradiating the fluorophore with the excitation light is interrupted.
It takes time for the light emission intensity of fluorophore and phosphor to be saturated. In particular, the light emission intensity of the phosphor takes more time to be saturated than the light emission intensity of the fluorophore does. For this reason, the image reading apparatus needs to irradiate sheet with the excitation light until the light emission intensity of the fluorophore and the phosphor is saturated, which may cause a decrease in throughput.